Battle Arena is an online mode that allows people to battle with each other in an arena. In the Battle Arena, a room can be created for friends to join. When joined, players must first choose a character before being displayed on the arena. If one wants to switch their character or anticipated stage, they must leave the line and once done, rejoin the line in the very back. The rules of the arena are preset before the arena is opened, and once it is, they can only be changed by way of the owner shutting down the arena and reopening it with new rules.
Once a match is over, in a 1v1 arena, one person will be sent to the back of the line, while the other stays. In an arena with 3 or more people in the ring at once, the system is slightly different. If 1 person is waiting in line, only one person will have to leave the ring after the match is over, while if two or more are waiting in line, two people will leave the ring after the match is over. In the arena settings menu, the owner can change it to either have it set to winner stays or loser stays.
In the Battle Arena, players are represented by tokens. The Tokens display the character the player has chosen for their Smash tag. The main area is the line of people waiting to go into battle. The ring in the middle has player tokens clashing to represent an ongoing battle. The player can select their character or anticipated stage by leaving the line, and can also place their token in the stands to the left of the ring to spectate the next match that starts, but doing so makes the player lose their place in line.
Battle Arenas created by friends are visible on their friend lists. People who aren't registered as friends can still join other player's Battle Arenas, however they would need to search for the Arena ID. Furthermore, Battle Arenas can be set with a password so that only those who have the password can enter.
In the 3.1.0 software update, amiibo were made to be usable in the Battle Arena.